1. Field of the Invention
The present invention concerns a radiator diaphragm, i.e., a device for collimation of x-rays of the type having at least one masking element that is adjustable between two end positions. Furthermore, the invention concerns a method to control this radiator diaphragm or collimator device and an x-ray CT device embodying such a radiator diaphragm or collimator device according to the invention.
2. Description of the Prior Art
Collimation is the procedure for directing light beams or x-rays to propagate in a parallel manner. A lens that directs the light of a punctiform (in terms of its focal point) light source parallel to the optical axis of the lens is used for collimation of light rays. This parallel beam can be used as a replacement for a light source at infinite distance. Diaphragm devices (known as radiator diaphragms) are typically used for collimation or of x-rays in CT apparatuses (computed tomography apparatuses). Many variants and processes are hereby known.
The x-ray fans (can beams) are gated in a direction that is usually designated as the z-direction by the collimation, so that only the rays that can be evaluated by the acquisition system (DMS—data measurement system) irradiate the patient. This is intended to prevent the patient from being exposed to a higher radiation dose in the examination that would be absolutely necessary. The reason for this is in that the employed x-rays can cause mutations in a living organism. In principle it should be ensured that only the minimum possible radiation dose is used.
The collimation width of a slot through which the generated x-rays pass is adjusted before beginning an examination of a subject known as the “scan”. The collimation width remains constant during the “scan”. This means that the slot produced by adjustment in the diaphragm must generally be displaced in the z-direction in order to compensate for position changes of the focus, for example due to thermal (heating) effects.
The principles of z-collimation or gating are presented in the following. Two prevalent methods have conventionally been used in order to collimate the beam fans in the z-direction. In the first method two individually movable diaphragm jaws or plates are used that are mechanically or electronically coupled with one another. In this variant the electromechanical effort is relatively high. In contrast to this, a high flexibility in the adjustment of desired or required collimation width is advantageous. A z-regulation to compensate for thermal effects is possible without problems. The second method uses a movable plate with multiple slots of different widths. The collimator widths are thus predetermined in a fixed manner by the slot widths. In comparison to the first cited method, the electromechanical effort is very slight. A z-regulation for compensation of thermal effects is likewise possible.
Furthermore, radiator diaphragms can be differentiated between static and dynamic collimation. In static collimation, the adjustment ensues at the beginning of the examination and is not changed or adapted in the course of the examination. The patient is thereby subjected to an unnecessary beam exposure at the beginning or at the end of the scan, since not all measurement data can be reconstructed in a manner that is effective for the image. In dynamic collimation, the beam diaphragms are dynamically regulated. In order to not reduce the dose used for the scan and acquisition process at the beginning and end of a spiral scan, the radiator diaphragm can dynamically gate or mask the scan process.
In radiator diaphragms with diaphragm jaws, the individual diaphragm jaws conventionally are able to respectively mask only half of the beam fan. In United States Patent Application Publication No. 2006/0039536, a dynamic collimation is described in which each diaphragm jaw can respectively mask only half of the beam fan. In this type of collimation, a diaphragm jaw (collimator jaw) is opened or closed in the respective first or last rotation of a spiral scan. A dynamic collimation only on one side is thereby achieved, and thus in principle only one part of the unused dose is reduced. The collimation system described in that published application employs eccentric rollers as diaphragm jaws. Given the use of rollers, the space requirement for the masking region and the designed space height is increased.
By expanding the z-masking of CT apparatuses, the proportion of unused dose at the beginning and end of a spiral scan (helical scan) is significantly increased. This leads to an unnecessary radiation exposure for the patient at these points in time.